1. Field of the Invention
The present invention relates to a multi-step process for converting refuse-derived fuel (RDF) or biomass into ethers and/or alcohols suitable for blending into a clean burning, reformulated gasoline. The steps comprise: fast pyrolysis of refuse-derived fuel or biomass to oil vapors; catalytic cracking of the oil vapors to maximize the yield of mixed olefins; conversion of the mixed olefins to isobutene and isopentene; and reaction with alcohol and/or water with the isobutene and isopentene to form the corresponding ethers. These ethers, inclusive of methyl tertiary butyl ether (MTBE), when added to gasoline, provide a distinct lowering of unburned hydrocarbons and carbon monoxide in the exhaust of vehicles burning gasoline, and this is of considerable importance in view of the fact that the U.S. Congress in the Clean Air Act of 1990 mandated the use of oxygenated fuels in areas that have had severe air pollution problems. The inclusion of these ethers as a component of gasoline are also important for the reason that, the Environmental Protection Agency has ordered the reduction of the vapor pressure of gasoline produced for use in the summer, and refineries complying with these orders are left with a surplus of butane, which requires petroleum refineries to install facilities to convert the surplus butanes into MTBE or other olefins convertible to ethers similar to MTBE for use in gasoline.
2. Description of the Prior Art
Biomass is a potentially inexhaustible raw material for biofuels as well as a stable and independent alternative to petroleum, and this is particularly true during an oil crisis. Biofuels can cover a wide range of liquid fuels that may be produced by either biochemical or thermochemical conversion processes, and the thermochemical conversion of biomass to liquid fuels can occur indirectly by gasification, or directly by pyrolysis or liquefaction. Biomass pyrolysis and the subsequent upgrading of pyrolysis vapors to gasoline-like hydrocarbons has recently received considerable attention; however, an inherent problem associated with this process is removing oxygen in biomass and increasing its hydrogen content to form liquid fuels. This difficulty can be correlated to an effective hydrogen index (EHI), which is defined by Haag et al., "Catalytic Production of Aromatics and Olefins from Plant Materials" in Prepr. Div. Pet. Chem., Am. Chem. Soc. 25, 650-656 (1980). where H, O, N, S, and C are the relative numbers of atoms of hydrogen, oxygen, nitrogen, sulfur, and carbon in the feedstock.
In the case of cellulose, which has an empirical formula of (C.sub.6 H.sub.10 O.sub.5).sub.n, it should be observed that the EHI equals zero. Hardwood (e.g., C.sub.6 H.sub.9 O.sub.4).sub.n ! and refuse-derived fuel (RDF) (e. g., C.sub.15 H.sub.24 O.sub.8).sub.n ! have EHIs of approximately 0.2 and 0.5, respectively.
In biomass pyrolysis, in which no reducing agent such as H.sub.2 or CO is present, oxygen must be removed as CO.sub.2 or CO in order to increase the hydrogen content of the products, J. P. Diebold et al., "Biomass-to-Gasoline (BTG): Upgrading Pyrolysis vapors to Aromatic Gasoline with Zeolite Catalysts at Atmospheric Pressure" in Pyrolysis Oils from Biomass, ACS Symposium Series 376, 264-276, ed. E. J. Soltes and T. A. Milne, Washington: ACS, 1988.
Because biomaterials are structurally and chemically complex, biomass thermochemical conversion processes can result in complex and unstable species and fractions. For example, about 230 compounds have been reported to be present in the pyrolysis products of softwood and hardwood, E. J. Soltes et al., Organic Chemicals from Biomass 64, ed. I. S. Goldstein, Boca Raton, Fla.: CRC Press, 1981. Upgrading of these products to stable and more valuable fuels and chemicals represents a new challenge to catalysis, New York, Chen et al., Shape Selective Catalysis in Industrial Applications, New York: Marcel Dekker, 1989.
Further, while pyrolysis products can be thermally cracked to form about 15% by weight C.sub.2.sup.+ hydrocarbons, large amounts of methane and hydrogen are also produced, and this lack of specificity has led to the abandonment of the thermal cracking approach and to an increased ongoing effort in the catalytic cracking area.
Mixed polyolefin plastic scraps are converted to low pour point oils by thermal cracking in the liquid phase followed by catalytic conversion of the vapors over a zeolite catalyst, in U.S. Pat. No. 4,851,601. The process in that patent is limited to only mixed polyolefin plastics as feedstocks, and it is indicated that halogenated polymers such as polyvinyl chloride should not be used in order to avoid catalyst deactivation. Thermal cracking of the plastics is carried out in the liquid phase at temperatures between 420.degree.-470.degree. C. The second stage catalytic conversion reaction is carried out at a temperature range of 250.degree.-340.degree. C. In the second stage catalytic conversion, the weight hourly space velocities (WHSVS) preferred are between 0.75-1.0. The hydrocarbon oil product has a boiling range above 165.degree. C. and a low pour point. Product analysis shows 38.4% paraffins, 54.7% olefins, and 4.5% aromatics, and most of the hydrocarbons are in the C.sub.5.sup.+ range.
U.S. Pat. No. 4,308,411 utilizes ZSM-5 catalysts in order to upgrade biomass pyrolysis vapors. In particular, the process is directed to conversion of organic waste (cellulosic fraction of municipal solid waste) to hydrocarbons. The pyrolysis is conducted at a temperature of at least 300.degree. C. to form a mixture of oxygenates, followed by deoxygenation using a zeolite catalyst. More specifically, the examples deal with pure model oxygenates such as .alpha.-methylglucoside (C.sub.7 H.sub.12 O.sub.5) and tetrahydrofurfuryl alcohol (THFA, C.sub.5 H.sub.10 O.sub.2). Both of these oxygenates are slightly hydrogen rich feedstocks (EHI's of 0.29 and 1.2). The catalytic reactor temperature range is between 275.degree.-425.degree. C. with a WHSV of 1-10 hr.sup.-1. The catalyst is an intermediate pore size zeolite having a pore diameter of between 5.3-6.0 .ANG.. The products obtained from this process of conversion include 15% char, 11% catalyst coke, 20% gaseous products, 24% water soluble organics, and 30% organic liquids. Upon analysis, the organic liquids were found to be 52% aromatic.
A process for catalytic conversion of biological materials to liquid hydrocarbons is disclosed in U.S. Pat. No. 4,300,009. The biological materials have an effective hydrogen index (EHI) of at least 1.0 and preferably greater than 1.3. The examples in this patent are pure biological feedstocks such as natural rubber latex, limonene, squalene, corn oil, peanut oil, castor oil, jojoba oil and tall oil. The catalytic reactor temperature range is between 300.degree.-650.degree. C. with a reactant WHSV of 0.2-20.0 hr.sup.-1. The catalyst is a crystalline zeolite having a pore diameter greater than 5.0 .ANG.. The products obtained include paraffinic, olefinic, and aromatic hydrocarbons; however, there is no selectivity towards one specific product or one group of hydrocarbons.
Chen et al. disclose working with model compounds (acetic acid and methylacetate) and affecting pyrolysis liquid upgrading in a fluidized bed apparatus in "Fluidized Bed Upgrading of Wood Pyrolysis Liquids and Related Compounds" ACS Symposium Series 376, 277-289. A ZSM-5 catalyst in a silicaalumina binder is used, at a temperature of 410.degree. C. and a WHSV of 1-2 hr.sup.-1. The products obtained were 0.7% CO, 10.5% CO.sub.2, 70.7% water, 2.5% C.sub.1 -C.sub.4 hydrocarbons, 6.0% C.sub.5.sup.+ hydrocarbons, and 9.6% coke.
None of the foregoing prior art processes uses refuse derived fuel (RDF) which contains all plastic waste including polyvinyl chloride (PVC), in addition to organic wastes; and these processes would of necessity incur extra cost in order to separate these plastics. Moreover, only a limited WHSV is utilized in the processes of the prior art references, and this results in lower selectivity towards olefins.
Further still, there is no indication in the prior art that the feedstock may have an EHI as low as zero, and the relatively low catalytic reactor temperatures of the prior art results in higher coke yields and lower yields of gaseous products, without any unique product selectivity of lower molecular weight C.sub.2 -C.sub.5 olefins.
As a further encumbrance, none of the prior art processes are directed to pyrolysis and upgrading performed in the same system; they are therefore less capable of preventing any possible repolymerization or condensation reactions that would result in lower coke formation on the catalysts.
There is a need in the general art of pyrolysis and upgrading of biomass and wastes to provide means for conversion of refuse-derived fuels which contains all plastic wastes (inclusive of polyvinyl chloride) in addition to organic wastes, so that no extra costs are encountered for separation of these plastics, and whereby chlorine in the polyvinyl chloride is converted to HCl without causing catalyst deactivation.
There is a further need in the general art of pyrolysis and upgrading of biomass and wastes to utilize a wider range of space velocities that are more favorable in large scale industrial operations and result in higher selectivity towards olefins, and whereby selectivity towards low molecular weight C.sub.2 -C.sub.5 range olefin products is achieved by using an appropriate catalyst.
A yet further need in the general art of pyrolysis and upgrading of biomass and wastes is the need to utilize a feedstock having an effective hydrogen index (EHI) as low as zero, high catalytic reactor temperatures in the range of about 450.degree.-550.degree. C. in order to provide lower coke yields and higher yields of gaseous products and at the same time provide unique product selectivity to low molecular weight C.sub.2 -C.sub.5 olefins, by selection of appropriate zeolite catalysts.
There is a yet further need in the general art of pyrolysis and upgrading of biomass and wastes to achieve pyrolysis and upgrading in the same system, in order to prevent any possible re-polymerization or condensation reactions, in order to provide lower coke formation on the catalyst, while obtaining higher yields of hydrocarbons and lower yields of water.